The present invention relates to a continuous method for making polyetherimide, based on the melt polymerization of an aromatic bis(ether anhydride) and an organic diamine utilizing an extruder reactor.
Prior to the present invention, as shown in Takekoshi et al, U.S. Pat. No. 4,011,198, incorporated herein by reference and assigned to the same assignee as the present invention, there is provided a method for making polyetherimides by effecting reaction at melt polymerization temperatures between an aromatic bis(ether anhydride), or "organic dianhydride" and an organic diamine in an extruder. The organic dianhydride has the formula, ##STR1## and the organic diamine has the formula, EQU H.sub.2 N--R.sup.1 --NH.sub.2, (2)
where R is a member selected from ##STR2## and divalent organic radicals of the general formula, ##STR3## X is a member selected from the class consisting of divalent radicals of the formula, ##STR4## y is an integer from 1 to 5; and R.sup.1 is a divalent organic radical selected from the class consisting of aromatic hydrocarbon radicals having from 6 to about 20 carbon atoms and halogenated derivatives thereof, alkylene radicals having from 2 to about 20 carbon atoms, cycloalkylene radicals having from 3 to about 20 carbon atoms, from C.sub.2 to about C.sub.8 alkylene terminated polydiorganosiloxanes and divalent radicals of the general formula, ##STR5## Q is a member selected from the class consisting of EQU --O--, --S--, C.sub.x H.sub.2 x,
x is an integer from 1 to 5 and n is 0 or 1.
Further improvements in the melt polymerization of mixtures of organic dianhydride of formula (1) and organic diamine of formula (2) to make polyetherimide are shown by Banucci et al U.S. Pat. No. 4,073,773, incorporated herein by reference and assigned to the same assignee as the present invention. Banucci et al show the introduction of a powdered mixture of organic diamine and aromatic bis(ether anhydride) into an inlet opening of a screw extruder. The powdered mixture of organic diamine and aromatic bis(ether anhydride) were blended in certain proportions and attempts were made to maintain the powdered mixture as uniform as possible. The preformed powdered blend was passed through a first extruder zone maintained at a low temperature to a zone where the mixture was melted and water of reaction was removed.
Although the methods of Takekoshi et al and Banucci et al provide procedures for making polyetherimide in an extruder by melt polymerization of a mixture of organic dianhydride of formula (1) and organic diamine of formula (2), steady state conditions required for continuous production of polyetherimide by melt polymerization are often difficult to achieve. The temperature profile of monomer solids initially fed into the extruder have to be carefully monitored prior to the venting of water of reaction to avoid plugging of the monomer feed in the extruder. The conversion of the solid monomers to the molten state can occur in Banucci et al at the mass transfer stage where water is evacuated. This procedure can interefere with the creation of steady state conditions in the extruder and optimum polymerization conditions for making polyetherimide.
A significant feature of the present invention is based on the discovery that steady state conditions in polyetherimide formation can be achieved by introducing a mixture of organic dianhydride of formula (1) and organic diamine of formula (2) into an extruder reactor and thereafter rapidly converting the monomer solids to the liquid state by internal viscous heating. Viscous heating is a frictional heating resulting from intermolecular contact between molecules under a high rate of shear. For example, mechanical kneading of material can be used. The resulting liquid feed mixture is thereafter conveyed to a mass transfer stage resulting in the elimination of water of reaction, followed by an advanced polymerization stage. Conversion of the dry-blended feed mixture to the liquid state is achieved by viscous heating at a temperature in the range of from about 50.degree. C. to 350.degree. C. resulting from the mechanically working of the material during conveyance. Mass transfer of water of reaction is enhanced at the elevated temperature resulting from the viscous heating. The resulting melt pool is thereafter subjected to high shear and intensive mixing at temperatures in the range of from about 250.degree.-400.degree. C. and at elevated pressure. To improve the polymerization rate, there optionally can be included one or more additional mass transfer stages.